Known methods of measuring coefficient of friction consist of force transducers or gauges and linear or rotational motors for dragging test materials and/or tools across test substrates. Examples include tire friction on road services, as disclosed in U.S. Pat. Nos. 5,132,906 and 6,015,192 tire and road surface friction applications. Additional inventions include devices for testing lubricity of fluids in drilling muds for oil wells as in U.S. Pat. No. 6,105,415 and fuels, as in GB2404032A, and U.S. Pat. No. 7,013,713. Many of these methods and devices are tribology science instruments related to wear on impinging surfaces. Further, the Horizontal Slip Meter is a typical device as detailed in ASTM F609 for the friction measurement of foot wear, for example, on dry walkway surfaces and the High Frequency Reciprocating Rig is a friction and wear instrument typically used for fuels and lubricants. ASTM D1894 describes static and kinetic coefficient measurements for plastic film and sheeting.
Various tribology test methods teach the use of single or multiple balls, pin on disc, ball on disc, plate on disc for estimates of friction on wet or dry surfaces with or without surface modifications. A four-ball wear and friction test apparatus is taught in U.S. Pat. No. 6,857,306. Regardless of the test configuration and test geometry, the primary objective is not one that focuses on the total motion of the test material. Typical data results show Force vs Speed or Stribeck curves of coefficient of friction. Data output appears to be based on steady state equilibrium measurements or the assumption of steady state. Friction factors as a function of speed using a ball on triple pad device is disclosed in U.S. Pat. No. 8,342,032.
In many mechanical and biological applications, the full motion of the test system is relevant and steady state is never fully achieved. For systems that include compliant, deformable surfaces, the full motion is frequently critical in understanding the benefit of lubricants, surface treatments, polishes, etc. This includes not only the static and dynamic friction coefficients, but the deformation of the soft substrates before bulk motion is observed. Examples of this include the application of a topical to a body surface. This can be lip treatment/colorant, a lotion or cream or gel to a body surface, a lubricant for medical examinations, rash treatments, optical treatments, etc. The initial resistance to motion of the physiological surface is critical to the application of the product as this can cause discomfort or damage to the skin surface. Further examples include the shear resistance of adhesives to shearing motion. Ultimate failure of this surface can be complex, dependent on the strength of the substrate/adhesive bond, the thickness of the adhesive layer and its stiffness, humidity, temperature, etc. An entire measurement profile from the application of a normal and horizontal force, its creep, yield and dislocation falls outside the scope of most coefficient of friction, lubricity, or tribology instruments. Another example is related to the motion of the eye in the absence and presence of lubricants and other medicaments or in the presence of corrective contact lenses. Nairn and Jiang in “Measurement of the Friction and Lubricity Properties of Contact Lenses” present results in terms of Stribeck curves of the coefficient of friction as a function of Sommerfeld Number and show both normal and frictional force curves as a function of time. The data shows that the emphasis is not on the repetitive onset of blinking eye movement nor on the transient nature of the motion. Friction Abrasion Analyzer Triboster (TS-501) by Kyowa Interface Science Co manufactures a friction device that operates in the horizontal plane in a linear or reciprocating manner. Abrasion data output is friction coefficient as a function of cycles employing flat, point, facet, line contact parts. Data output is in terms of coefficient of friction as a function of cycles. U.S. Pat. No. 8,342,032 teaches a Tribology Device for Assessing Mouthfeel Attributes of Foods. This ball-on-three-plates rotational tribology device uses a thermoplastic polymer as an integral part and measures friction factors as a function of sliding speed.
Regardless of the nature of the testing materials or test substrates, the lubricity metrics and tribology have been focused on coefficient of friction measurements and not the total deformation/resistance curves. In fact, compliance of the substrates is often experimentally minimized, as cited by Michielsen in working with fabric matting and foams. The coefficient of friction is recited as data output from the device. Further, coefficient of friction of soft contact lenses is measured by Rennie et al. This comprehensive study provides coefficient of friction measurements as a function of numerous variables without reference to the time record of the force measurements. The lack of robust data acquisition appears to be the principle deficiency of current methodologies and devices. Lubricity, as a measure of skin feel, was measured as a coefficient of friction between two surfaces with a reciprocating teflon blade by Hughes, Lvovich, Woo, Moran, Suares, and Truong. The change in coefficient of friction during spreading with absorption of the emollient and evaporation is not measured. The Nanovea Tribometer using alumina ball and glass slides, measures coefficient of solutions and provides time based force results, using a flat or sphere shaped indenter and rotational disk. Wear coefficients are calculated with varying time, contact pressure, velocity, and other variables.